The present invention concerns a pasty dental material, which hardens to a high-gloss polishable mass in the presence of an initiator, formed of a polymerizable bonding agent and a finely divided filler as the essential components. The material contains as a bonding agent at least one polymerizable methacrylate and an optionally silanizable, new filler based on functional polysiloxanes. It may also contain initiators to trigger polymerization, other fillers, such as finely ground glass types, highly dispersed silicas or previously produced polymers, pigments and stabilizers. Other additives such as softeners or impact strength improvers may also be contained in the composition.
The term "dental material" as used herein means, for example, filling materials for carious defects or other dental defects in the mouth, inlays, crown and bridge materials, veneers, sealing and protective coatings, plastic adhesives to fix inlays or crowns and bridges, stump build-up materials, denture materials as well as masses for the production of artificial teeth.
Standard dental masses of the above mentioned type contain at least one monomeric ester of methacrylic acid, but in most cases a mixture of several of such esters. Suitable monofunctional esters of methacrylic acid are e.g. methylmethacrylate, ethylmethacrylate, isopropylmethacrylate, n-hexylmethacrylate and 2-hydroxyethylmethacrylate.
Recently, multi-functional esters of methacrylic acid with a higher molecular weight have also been used frequently, such as ethylene glycol dimethacrylate, butandiol-1,4-dimethacrylate, triethylene glycol dimethacrylate, dodecandiol-1,12-dimethacrylate, decandiol-1,10-dimethacrylate, 2,2-bis-[p(.gamma.-methacryloxy-.beta.-hydroxypropoxy)-phenyl]-propane, the diaduct of hydroxyethylmethacrylate and trimethylhexamethylene diisocyanate, the diaduct of hydroxyethylmethacrylate and isophoron diisocyanate, trimethylolpropanetrimethacrylate, pentaerythritetrimethacrylate, pentaerythritetetramethacrylate and 2,2-bis[p(.beta.-hydroxy-ethoxy)-phenyl]-propanedimethacrylate (bis-GMA).
The materials for dental use may, depending on the application purpose, be hardened in different ways. Dental filling materials exist in the form of photo-hardening as well as self-hardening (autopolymerizing) masses. The photohardening masses contain photoinitiators such as benzoinalkylether, benzilmonoketals, acylphosphinoxide or aliphatic and aromatic 1, 2-diketo-compounds such as e.g. camphorquinone as well as polymerization accelerators such as aliphatic or aromatic tertiary amines (e.g. N,N-dimethyl-p-toluidine triethanolamine) or organic phosphites and harden when irradiated with UV or visible light.
The self-hardening materials consist as a rule of a catalyst paste and a base paste of which each contains a component of a redox system and which polymerize when the two components are mixed. One component of the redox system is in most cases a peroxide, such as e.g. dibenzoylperoxide, the other is in most cases a tertiary aromatic amine, such as e.g. N,N'-dimethyl-p-toluidine.
Other dental materials such as prosthesis plastics or plastic masses for the production of artificial teeth polymerize under heat application. The initiators in those cases are as a rule peroxides such as dibenzoylperoxide, dilaurylperoxide or bis(2,4-dichloro-benzoylperoxide).
Dental materials also contain as a rule pigments which--added in small amounts--act to match the color of the dental masses with the various shades of natural teeth. Suitable pigments are e.g. black iron oxide, red iron oxide, yellow iron oxide, brown iron oxide, cadmium yellow and orange, zinc oxide and titanium dioxide.
Dental materials further contain mostly organic or inorganic fillers in order to avoid a shrinking of the volume of the plastic mass during polymerization. Pure monomeric methylmethacrylate for instance shrinks during polymerization by about 20 vol %. By adding about 60 weight parts of solid methylmethacrylate-pearl polymer the shrinking may be reduced to about 5-6 vol % (see German Patent 24 03 211).
Other organic fillers are obtained by producing a polymer which consists essentially of esters of methacrylic acid and either is or is not cross-linked. This polymer optionally contains surface-treated fillers. If it has been produced as a polymer it may be added to the dental material in this form; if on the other hand it was produced by solventless polymerization in compact form it must be ground into a so-called splinter polymer before being added to the dental material.
Frequently used preformed polymers are in addition to the already mentioned filler-containing pearl and splinter polymers homopolymers of methyl methacrylate or, preferably non-cross-linked, copolymers of the methyl methacrylate with a low content of esters of the methacrylic acid or acrylic acid with 2 to 12 carbon atoms in the alcohol component, preferably in the form of a pearl polymer. Other suitable polymers are non-cross-linked products based on polyurethanes, polycarbonates, polyesters and polyethers.
Inorganic fillers are e.g. ground glasses or quartz with average particle sizes between 1 and 10 micrometers as well as highly dispersed silica with average particle sizes between 10 and 400 nm.
The glasses are preferably aluminumsilicate glasses which may be doped with barium, strontium or rare earths (see German Patent 24 58 380).
It should be noted in regard to the finely ground quartz or the finely ground glasses and the highly dispersed silica that the inorganic filler is as a rule silanized prior to the mixing with the monomers in order to achieve better bonding to the organic matrix. For this purpose the inorganic fillers are coated with silane coupling agents which in most cases have a polymerizable double bond for reaction with the monomeric esters of the methacrylic acid.
Suitable well known silane coupling agents for this purpose are e.g. vinyltrichlorosilane, tris-(2-methoxyethoxy)-vinylsilane, tris-(acetoxy)-vinylsilane and 3-methacryloyloxypropyltrimethoxysilane.
The initially mentioned, recently used monomers with higher molecular weight also result in a decrease of polymerization shrinkage. To these monomers the described inert inorganic finely ground glasses or organic fillers or mixtures thereof are added up to 85 wt %, which results in a further reduction of the shrinkage to ca. 1 vol %.
The inorganic fillers not only result in a decrease in polymerization shrinkage but in addition in a significant strengthening of the organic polymer structure.
This strengthening also shows itself in an improvement of mechanical characteristics and in an increase in wear resistance (R. Janda, Quintessenz 39, 1067, 1243, 1393; 1988). Good mechanical characteristics and high wear resistance are important requirements which must be fulfilled by a hard dental compound which is intended as a long-term replacement for lost hard natural dental substance.
But in addition to strengthening characteristics, the filling compounds also must fulfill other material parameters. An important parameter in this context is the polishability. High gloss polishability is of significant importance for filling materials and crown and bridge materials for at least two reasons:
A high-gloss and completely homogenous surface must be required of the filling material for aesthetic reasons, so that the filling cannot be distinguished from the surrounding, absolutely smooth, natural tooth enamel. This high-gloss filling surface also must retain its characteristic for a long period. PA1 A highly smooth filling surface is also important so that plaque or discoloring media cannot attach themselves.
However, it has now been shown that the previously described finely ground quartz or glass fillers have good strengthening characteristics, but do not fulfill requirements in regard to their polishability. It has therefore been attempted to grind these fillers even finer in order to obtain more homogenous surfaces. However, physical grinding methods are limited, so that average grain sizes below 1 .mu.m are hard to achieve.
When highly dispersed silica (average particle size 10 to 400 nm) was used as a filler in dental masses (German Patent 24 03 211) it was surprisingly shown that these fillers were able to achieve a significant improvement in polishability. Disadvantages of the highly dispersed silica result from its great thickening effect, so that today, as a rule, filling amounts above 52 wt % cannot be obtained, unless insufficient processing technology characteristics are tolerated.
The materials filled with highly dispersed silicic acid also showed clearly lower stabilities and hardness than those filled with quartz or finely ground glass.